The diversity of the cytochrome P-450 in human tissues, i.e. placenta, monocytes and lymphocytes was investigated using monoclonal antibodies to 3-methylcholanthrene (MC) or phenobarbital (PB)-induced rat liver cytochrome P-450. Monoclonal antibodies to rat liver cytochrome P-450 not only inhibited the aryl hydrocarbon hydroxylase (AHH) of MC-induced rat liver microsomes but also inhibited AHH of human placenta and lymphocytes. The 7-ethoxycoumarin deethylase activity of human placenta and lymphocytes was also inhibited. The degree of enzyme inhibition by monoclonal antibodies was different for different individuals and tissues. Monoclonal antibodies may be applied usefully in analyzing the multiplicity, distribution and function of the cytochromes P-450 in human tissues.